Does he like me or is it just because he's extremely shy? ?

He was probably just trying to be sociable with the height thing ... How to get him to open up? That is something only he can choose to do or not. But, since you cannot force this on him, you could encourage that you'd benefit from a study group, the line being "let's ace this summer class". Just go easy though, don't force it on him but do begin acting like you've decided it's a deal underway. "So could we meet up at the library at 4 then, just for half an hour at a time is lots" is one line. And, "I could use a sub after this session, how about you?" could be for another day. Just small amounts every couple days, leave some days up to him or make them as days off, so he's not overwhelmed. By the end of this course ... you should know how you and he are doing in and out of class. Just don't over-do it till end of summer school, to give him a chance to think about you and figure out what's going on, etc.